The present invention relates to an automatic focusing apparatus for a microscope.
Various techniques for sensing the focal point of an optical instrument, such as an microscope, have been proposed. One of them is an automatic focusing apparatus using an optical path difference method.
In the automatic focusing apparatus for a microscope using the optical path difference method, the following two points particularly become problems:
1. Since some objectives of microscopes have an extremely low magnification of less than .times.1 and others have a high magnification of .times.100, automatic focusing control suitable for the magnification range of such objectives.
2. When the contrast of a specimen, an object to be observed, is low, the signal-to-noise ratio gets worse in the focused position of a defocus signal indicating the degree of focusing of the specimen (object) and the accuracy of focusing decreases.
These problems will be explained below. The characteristic of the contrast value (defocus signal) with respect to the change of the magnification of the objective is shown in FIG. 1A to FIG. 1C.
In FIG. 1A to FIG. 1C, each vertical line represents the contrast value of the specimen and each horizontal line represents the position of the stage in the direction of Z (in the direction of optical axis).
As shown in FIG. 1A to FIG. 1C, when the magnification of the objective changes from high to low or from low to high, the optical path difference with respect to the position of the surface of the specimen is inversely proportional to the square of the magnification of the objective. Specifically, when the magnification of the objective is low, the contrast characteristic of the contrast value in the front focused position separates too far from that in the back focused position as shown in FIG. 1A, as compared with an ideal case shown in FIG. 1B. As a result, the contrast value in the actual focused position decreases, which decreases the signal-to-noise ratio.
In contrast, when the magnification of the objective is high, the characteristic of the contrast value in the front focused position is too close to that in the back focused position as shown in FIG. 1C, which makes smaller the difference between the front focused position and the back focused position. As a result, the optical path difference in effecting automatic focusing control is less likely to appear.
Specifically, with the conventional automatic focusing apparatus for a microscope, because the signal-to-noise ratio of the contrast value of the specimen gets worse at low magnifications and the optical path difference is less likely to appear, both of these factors make automatic focusing control difficult.
When the specimen has a low contrast value, a focus judgment is made in a region where the contrast value of the specimen is lower than the maximum value because of the characteristic of the optical path difference method, which leads to a decrease in the accuracy of focusing. It is clear that the lower the magnification becomes, the worse the accuracy of focusing.
To solve the above problems, in a focal point sensing apparatus disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 59-10913, a zoom mechanism operating in inverse proportion to the magnification of the objective is inserted in the optical path to offset the change of the optical path difference caused by the magnification of the objective.
Furthermore, in the case of an automatic focusing mechanism for a microscope disclosed in Jpn. Pat. Appln. KOKAI Publication No. 63-78113, a prism that changes the optical path difference according to the magnifica- tion of the objective is inserted in the optical path to suppress the change of the optical path difference in a similar manner.
Both of the automatic focusing apparatuses require an optical path difference changing mechanism, such as a zoom mechanism or a prism switching mechanism. This not only makes the microscope larger and raises costs but also needs the parfocalizing of the zoom mechanism and adjustment of the prism, which decreases the reliability of the microscope seriously.
Furthermore, in the case of a specimen with a low contrast value, some portions have high contrast values and others have low contrast values. Therefore, it is impossible to secure the accuracy of focusing by changing the optical path difference as described above.